Heretofore, certain materials such as metals and carbon materials have been used in fields wherein high electroconductivity is required. Among these, carbon materials are free from corrosion unlike metals, and are excellent in electroconductivity, thermal resistance, lubricity, heat conductivity, durability, etc.
Therefore, carbon materials have played an important role in various fields such as electronics, electrochemistry, energy and transportation equipment, etc. Also, composite materials based on a combination of a carbon material and a polymer material have made remarkable progress and permit high performance and high function in various materials. Particularly, the degree of freedom of mold workability is expanded due to a combination of a carbon material and a polymer material, and this is one reason why carbon materials have been developed in fields where electroconductivity is required.
Examples of the usage or application for the carbon material in which the electroconductivity is required, may include: electronic materials such as circuit boards, resistors, laminates, and electrodes; and various members such as heaters, members constituting heat-generating devices, and dust-collecting filter elements. In these applications, high thermal resistance is required together with an electro conductivity.
On the other hand, in view of environmental problems and energy problems, fuel cells have attracted much attention as clean power-generating devices, because they generate electric power by a reverse reaction of electrolysis using hydrogen and oxygen, and they produce no exhaust material other than water. Also, in this field, carbon materials and polymer materials have important roles.
The fuel cells can be classified into several kinds, depending on the kind of the electrolyte to be used therefor. Among such fuel cells, solid polymer electrolyte-type fuel cells can work at a low temperature, and therefore they are most useful for automobile or public or civilian uses. This type of fuel cell is constructed by stacking unit cells, each of which comprises, e.g., a polymer electrolyte, a gas diffusion electrode, a catalyst and a separator, and the fuel cell can attain high-output power generation.
In the fuel cell having the above structure, the separator is always in contact with water which is produced by the reaction for electric power generation. It is said that the operating (or working) temperature of the above solid-type fuel cell is about 80° C. However, in applications of the fuel cell wherein a long operating time is expected, the separator is required to have a thermal resistance (particularly, hydrothermal resistance) which can bear the use thereof for a long period.
In addition, the separator to used for such a purpose to partition the unit cells usually has at least one flow channel (or groove) to which a fuel gas (hydrogen) and an oxidant gas (oxygen) are supplied, and from which the produced water content (steam) is discharged. Therefore, the separator is required to have a high gas impermeability capable of perfectly separating these gases, and is also required to have a high electroconductivity to reduce the internal resistance. Further, the separator is required to be excellent in heat conductivity, durability, strength, etc.
To satisfy these requirements, the separator has been heretofore studied in view of both aspects of metal and carbon materials. Metals have a problem in the corrosion resistance thereof and therefore, an attempt has been made to cover the surface thereof with a noble metal or carbon. However, in such a case, a sufficiently high durability cannot be obtained and moreover, the cost for covering the metal is problematic.
On the other hand, a large number of carbon materials have been studied as carbon materials for fuel cell separators, and examples thereof include a molded article obtained by press-molding an expanded graphite sheet, a molded article obtained by impregnating a carbon sintered body with a resin and curing (or hardening) the resin, a vitreous carbon obtained by baking a thermosetting resin, and a molded article obtained by mixing a carbon powder and a resin and molding the resultant mixture.
For example, JP-A-8-222241 (Patent Document 1; the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a complicated process such that a binder is added to a carbon powder and mixed under heating, the mixture is CIP (Cold Isostatic Pressing)-molded, baked and graphitized, and the thus obtained isotropic graphite material is impregnated with a thermosetting resin and subjected to a curing treatment, and grooves are engraved therein by cutting.
JP-A-60-161144 (Patent Document 2) discloses a technique of impregnating a paper containing carbon powder or carbon fiber with a thermosetting resin, stacking and press-bonding the resultant papers, and baking the stacked body. JP-A-2001-68128 (Patent Document 3) discloses a technique of injection-molding a phenol resin into a separator-shaped mold and baking the molded resin.
The materials which have been subjected to a baking treatment as in these examples can exhibit a high electroconductivity and a high thermal resistance, but the baking takes a long time and the productivity is low, and these materials also have a problem that they have a poor bending (or flexural) strength. Further, when cutting these materials is necessary, the mass productivity is low and the cost is high and, therefore, these materials can hardly become popular in the future.
A molding method is considered as means being expected to bring high mass productivity and low cost.
The material applicable thereto is generally a composite of a carbonaceous material and a resin. For example, JPA-58-53167 (Patent Document 4), JP-A-60-37670 (Patent Document 5), JP-A-60-246568 (Patent Document 6), JP-B-64340 (Patent Document 7; the term “JP-B” as used herein means an “examined Japanese patent application”) and JP-B-6-22136 (Patent Document 8) disclose a separator comprising graphite, carbon and a thermosetting resin such as phenol resin. JP-B-57-42157 (Patent Document 9) discloses a bipolar separator comprising a thermosetting resin such as epoxy resin, and an electroconductive substance such as graphite, and JP-A-1-311570 (Patent Document 10) discloses a separator obtained by blending an expanded graphite and a carbon black with a thermosetting resin such as phenol resin and furan resin. In addition, JP-A 11-154521 (Patent Document 11) disclose a separator capable of preventing the deterioration in the use thereof at a high temperature by using the brominated epoxy resin as a fire retardant.    [Patent Document 1] JP-A 08-222241    [Patent Document 2] JP-A 60-161144    [Patent Document 3] JP-A 2001-068128    [Patent Document 4] JP-A 58-053167    [Patent Document 5] JP-A 60-037670    [Patent Document 6] JP-A 60-246568    [Patent Document 7] JP-B 64-000340    [Patent Document 8] JP-B 06-022136    [Patent Document 9] JP-B 57-042157    [Patent Document 10] JP-A 01-311570    [Patent Document 11] JP-A 11-154521
The above-mentioned conventional various kinds of cured products comprising thermosetting resins and carbonaceous materials do not have sufficient performances, in view of a high thermal resistance which is required in many applications such as electrodes, heaters, heat-generating device members, and fuel cell separators.
In addition, and particularly in the case of a fuel cell separator, it is required to have a hydrothermal resistance, in addition to a thermal resistance. However, the above-mentioned conventional cured products comprising thermosetting resins and carbonaceous materials do not have sufficient performances, in view of a high hydrothermal resistance which is required for the use of fuel cell separators. In other words, the thermosetting resin having an ester bond or urethane bond in the structure thereof can be hydrolyzed in some cases by hot water produced from the fuel cell. Accordingly, when the conventional cured product comprising a thermosetting resin and a carbonaceous materials is used in the case of automobiles and household electric appliances which are expected to be used for a long time, it is impossible to obtain a product having a sufficient durability.